Climate instability during the termination of the African Humid Period: a model study
by
Renssen, Hans
Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, The Netherlands
Coauthors: Victor Brovkin (Postdam Institute for Climate Impact Research, Germany), Thierry Fichefet (Institut d´Astronomie et de Géophysique Georges Lemaître, Université catholique de Louvain, Louvain-la-Neuve, Belgium), Hugues Goosse (Institut d´Astronomie et de Géophysique Georges Lemaître, Université catholique de Louvain, Louvain-la-Neuve, Belgium)

INTRODUCTION. Numerous palaeoclimatic data show that the early Holocene (~9-7 kyr BP, thousand year before present) was a relatively humid period in Northern Africa. During this phase, the African Humid Period (AHP), grasslands covered the Sahara/Sahel region, and many lakes and wetlands existed here (Prentice et al., 2000; Gasse et al., 2000). The humid conditions at this time are associated with a strengthening of the summer monsoon circulation, which in turn was due to an increase in the land-sea thermal contrast under influence of the relatively high summer insolation in the early Holocene (Kutzbach and Street-Perrott, 1985). Some palaeoevidence from ocean cores indicates that the AHP came to an abrupt end around 6 kyr BP (DeMenocal et al., 2000). This abrupt termination is linked to a positive biogeophysical feedback between vegetation and precipitation in the Sahara region: when precipitation is reduced, the vegetation cover decreases, thereby increasing the surface albedo, which in turn leads to a decrease in precipitation (Charney et al., 1975). Previous climate model experiments have reproduced the abrupt termination of the AHP, thereby showing the effectiveness of this biogeophysical feedback (Claussen et al., 1999). Thus, in short, around 6 kyr BP, a shift took place from a 'green' Sahara state to a desert state. Indeed, climate model studies have suggested that these two basic states could be stable in the Sahara/Sahel region (Claussen, 1998; Brovkin et al., 1998). In addition, some proxy data give evidence of rapid fluctuations between these green and desert states during the termination phase of the AHP (Fabre and Petit-Maire, 1988; Lézine et al., 1990), indicating that the dynamics of the coupled climate system are more complex than previously thought. To study these characteristics of the AHP in detail, we have conducted several numerical experiments with a coupled, three-dimensional climate model (Renssen et al., 2003a).

MODEL EXPERIMENTS. We have investigated the AHP termination in experiments performed with the ECBilt-CLIO-VECODE coupled atmosphere-ocean-vegetation model. The model consists of three components:

1) ECBilt, an atmospheric model (T21, three layers) based on quasi-geostrophic equations (Opsteegh et al. 1998), 2) CLIO, a oceanic general circulation model coupled to a comprehensive dynamic-thermodynamic sea-ice model (Goosse and Fichefet 1999), and 3) VECODE, a model that describes the dynamics of grassland and forest, and desert as a third dummy type (Brovkin et al. 2002.

The ECBilt-CLIO model reproduces reasonably well the modern climate (Goosse et al. 2001). It has been used to study meltwater-induced abrupt climate events during the early Holocene (Renssen et al., 2001, 2002), natural variability of the modern climate (Goosse et al., 2002) and future climate evolution (Goosse and Renssen, 2001; Schaeffer et al., 2002). ECBilt-CLIO-VECODE was used earlier to investigate the impact of global deforestation on the long-term stability of the ocean thermohaline circulation (Renssen et al., 2003b).

In our main experiment, we have forced the model with insolation (Berger, 1978) and atmospheric concentrations (Raynaud et al., 2000) of CO2 and CH4 for the last 9,000 years. All other boundary conditions were fixed at their 1750 AD values. The initial conditions were taken from an experiment in equilibrium with boundary conditions for 9 kyr BP. In the latter experiment, grasslands covered the Sahara in agreement with palaeobotanical data.

RESULTS. The simulated climate evolution is presented in Figure 1, and can be summarized as follows.

Phase 1: 9-7.5 kyr BP, 'green' state In the Western Sahara region, the model simulates from 9 to 7.5 kyr BP a 'green' equilibrium characterized by a mean annual precipitation of 290 mm/yr and a vegetation fraction of 70%. This green state is associated with a relatively strong land-sea thermal gradient, which strengthens the summer monsoons, leading to an increased transport of humid air towards the continent and enhanced convective precipitation over land.

Phase 2: 7.5-5.5 kyr BP, intermediate unstable state After 7.5 kyr BP, precipitation and vegetation concentration decrease to values of 210 mm/yr and 50%, respectively. In addition, the variability in vegetation fraction increases significantly (standard deviation is 9.2% for 9-7.5 kyr BP and 12.2% for 7.5-5.5 kyr BP). The time period separating the "green" spikes ranges from 110 to 370 years, which is similar to the lake-level fluctuations observed in high-resolution palaeodata from the Western Sahara.

Phase 3: 5.5 kyr BP to present, desert state After 5.5 kyr BP, the variability decreases substantially and the system moves towards a desert state. At 1 kyr BP, annual precipitation is as low as 60 mm/yr and vegetation fraction is only 10%.

The overall Holocene global change in precipitation and vegetation cover is shown in Figure 2, clearly indication of the extension of the simulated desertification in Northern Africa.

In addition to the main 9 kyr-long simulation, we have performed 4 additional sensitivity experiments of 200-yr duration to study the stability of the desert and green states through time. In these sensitivity experiments, we fixed in the first 100 years the vegetation in the Sahara region to either 100% desert or 100% grassland, after which the model was allowed to evolve freely during the remaining 100 years. All other forcings were kept constant. The four experiments can be characterized as follows: 9k-desert, 6k-desert, 6k-green, 0k-green. In the 0k-green experiment, the model quickly returned to the desert state after 100 years, showing that at under present-day conditions, only the desert state is stable in our model. In the 9k-desert experiment, on the other hand, the model returned to the green state, suggesting that this state is favoured under 9 kyr BP conditions. In the 6k-desert and 6k-green experiments, the model evolved towards an intermediate state with a vegetation cover between 45 and 60%. Thus, under 6 kyr BP conditions, the model has no clear preference for the green and desert states, explaining why, between 7.5 and 5.5 kyr BP, the stochastic variations in precipitation are able to induce transitions between the two states. The stability of the system in our model, as discussed above, is summarized in a diagram (Figure 3) that was constructed following the conceptual model analysis proposed by Brovkin et al. (1998).

To study the oceanic contribution to the variability in precipitation, we have conducted two additional sensitivity experiments with an identical design as 6k-desert and 6k-green, but without an interactive ocean (i.e. with 6 kyr BP sea-surface conditions prescribed). These two experiments showed that the high-frequency variations in oceanic surface temperature are responsible for only 20% of the variability in precipitation.

CONCLUDING REMARKS. According to our experiments, the Holocene evolution of the atmosphere-ocean-vegetation system may be summarised by the simplified diagram shown in Figure 4. Hypothetically, the climate-vegetation system possesses multiple steady states, desert and 'green'. Potential minima, marked by black balls, correspond to equilibria that are stable in the absence of perturbations. Precipitation fluctuations induced by large-scale atmospheric and oceanic variability perturb the stable state, and a positive feedback between vegetation and atmosphere amplifies external variability. Grey balls and arrows indicate the maximum range of system variations.

A. 9 kyr BP. The dynamical system has two steady states with a preference for the green state (deeper potential minimum). The system fluctuates in vicinity of the green state.

B. 6 kyr BP. The potential became equal for both states. The system fluctuates between desert and green states with a stronger variability than at 9 kyr BP. Consequently, 6 kyr BP is not a good period to test models in equilibrium experiments.

C. 0 kyr BP. The system underwent bifurcation as the green state lost stability and disappeared. Desert is the only steady state. Precipitation fluctuations are reduced in comparison with the two-well system (A and B). Similar transitions between multiple equilibria may have occurred during other orbitally forced transitions in the geological past.

The IPCC expects a significant increase in precipitation in the Western Sahara/ Sahel region in the 21st century (IPCC, 2001). This is likely to considerably increase the variability as it pushes the system in the direction of the 6 kyr BP state, with both the green and desert states being potentially stable.

ACKNOWLEDGEMENTS. T. Fichefet and H. Goosse are Research Associates at the National Fund for Scientific Research (Belgium). H. Renssen is supported by the Netherlands Organisation for Scientific Research N.W.O.

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Date received: November 28, 2003